1. Field of the Invention
The present invention relates to a circuit device in which a semiconductor device and a bus bar are bonded to a ceramic based board, and a manufacturing method thereof, and more particularly to a circuit device including a ceramic substrate, on a surface of which a wiring layer made of aluminum as a major component is formed, and a manufacturing method thereof.
2. Description of Related Art
Heretofore, there is a circuit device in which a plurality of semiconductor devices and bus bars are mounted on a board. An example of such board may include a ceramic substrate having ceramic material as a base body. For example, a board, called “DBA (Direct Bonded Aluminum) board”, in which an aluminum (Al) wiring layer is formed on both surfaces of an aluminum nitride (AlN) substrate is used in many cases. In order to bond the semiconductor device and others to the board by soldering, for example, the surface of the Al layer is typically coated with nickel (Ni) plating or other material plating.
In a power module using the above board, however, thermal shock attributable to heat generation of a semiconductor chip may be added to a solder layer during use. Accordingly, the solder layer may be cracked and the board is likely to be warped or broken due to a difference in coefficient of thermal expansion. In Japanese unexamined patent application publication No. 11-346037(1999), on the other hand, it has been proposed to bond a ceramic board to a bus bar part. This publication shows that such configuration can reduce stress resulting from a difference in coefficient of thermal expansion between the bus bar part and the ceramic substrate.
In Japanese unexamined patent application publication No. 2002-9190, a technique has been proposed to bond a metal lead to a metal circuit layer formed on a ceramic substrate by an ultrasonic welding technique. This technique enables the bonding without requiring heating. Accordingly, it is perhaps possible to reduce an influence on a boundary face between ceramics and metal and enhance the bonding reliability. This publication further states that materials of the metal circuit layer and the metal lead preferably include the same metal as a major component and also that a surface of the metal circuit layer may be coated with Ni plating.
However, the above mentioned conventional techniques have the following problems. The technique of the '037 publication needs the ceramic board bonded to the bus bar, thus leading to an increase in the number of parts and a complicated manufacturing process. In addition, a bonded part between the bus bar and the ceramic board may cause a thermal problem as in the above case.
The technique of the '190 publication can adequately avoid such problem that the bonded part is broken by heat, as compared with the case using the thermal bonding technique. Hence, the inventors of the present invention have tried to ultrasonically bond a copper bus bar to a typical DBA board coated with Ni plating. However, it can be hardly said that sufficient bonding property could be obtained. To find a cause thereof, the inventors examined the bonded part by peeling the bus bar from the substrate. As a result thereof, it was found that the bus bar and the substrate were not appropriately bonded to each other at a central portion of the bonded part. The reason thereof will be explained below.
A board 100 of a conventional circuit device subjected to the above inspection has a structure, as shown in FIG. 6, that a wiring layer 12 made of aluminum (Al) is formed on the surface of a ceramic substrate 11 made of AlN and others as main components, and further a Ni layer 13 is plated on the wiring layer 12. A semiconductor device 20 is bonded to the board 100 with solder 21. FIG. 6 shows a state of the board 100 on which the bus bar 30 is yet to be bonded.
Then, an ultrasonic horn 40 is set in contact with the bus bar 30 placed on the board 100 to perform ultrasonic bonding. At this stage, as shown in FIG. 7, the bus bar 30 is vibrated by the ultrasonic horn 40. The Al layer forming the wiring layer 12 of the board 100 is very soft as compared with the Ni layer 13. It was accordingly found that vibration for ultrasonic bonding caused so large deformation of the Al layer that the Ni layer 13 got stuck into the wiring layer 12. Specifically, energy of vibration could not appropriately be transmitted to a contact surface, i.e., the Ni layer 13.
Actually, the bus bar 30 and the board 100 were not appropriately bonded each other within most part of the bonded area as shown in FIG. 7. In the central portion of the bonded area, specifically, the bus bar 30 merely overlapped the Ni layer 13 without being bonded thereto. At an end portion of the bonded area, on the other hand, the Ni layer 13 was broken and pushed away, so that the bus bar 30 was bonded to the Al wiring layer 12. Thus, the actual area of the bonded part was small, which could not provide sufficient bonding strength.